The invention relates generally to a method and apparatus for reducing the size of utility company switch gear and protecting the sensitive components, such as generators and transformers used by power utilities, and in particular, to a current limiting method and apparatus for controlling the current through such generators and transformers during a fault condition.
In the utility industry, to protect current sensitive components such as generators and transformers, the utilities typically use passive current limiters without the ability to trigger upon the occurrence of a fault. Accordingly, these limiters limit the current only to values which are ten to one hundred times higher than the normal operating values for the equipment.
Since the expense and size of switch gear used by the utilities increases rapidly with fault current ratings, the utilities prefer to use the smallest switch gear which is commensurate with the application. The addition of fast operating current limiters, with tight overload specifications, would allow the utilities to split a high current bus without the requirement of using switch gear on each sub-bus which tolerates a high fault current. For these split-bus services, typically, for example, 240 kV, 4 kA lines, to operate successfully, the fault currents need to be limited to three or four times the normal load current. Thus, a limiter operating for 30 or 40 milliseconds, on a 240 kV line, must be able to sustain a 75-100 MJ energy load per fault.
When a fault occurs, for example a short circuit, the current through the current sensitive components can rise to fault values in less than 1 millisecond. Accordingly, a current limiter to be effective must be able to hold the current to less than several times normal values for at least about 30 milliseconds while the line circuit breaker operates. The line circuit breaker, while effective to decouple the fault from the current sensitive components, will not "kick in" for about 30 milliseconds once the fault occurs.
In the past, there have been two forms of current limiters used in utility power systems. These are inductive limiters and current limiting fuses. The current limiting inductors are air core inductors, having an inductance (L), placed in series with a circuit breaker. They limit current by generating a back voltage (V), due to the changing current (i), as a function of time (t) according to Lenz's and Faraday's laws: ##EQU1## The major advantages of inductive limiters are their simplicity, robustness, and low cost.
Current limiting fuses use a non-linear resistance, R, in series with the line, and the fuse vaporizes when the current exceeds a specified rating. These fuses accordingly are not reusable, and if reusable, would provide a back voltage depending upon the temperature of the limiter in accordance with Equation 2: EQU V=iR(i,T) (Eq. 2)
The advantages of a current limiting fuse are its low cost and positive action in the case of a fault.
Superconducting variants of these types of current limiters have been built. In addition, transformer type devices have also been built. The superconducting version of the current limiting fuse depends on a large change in resistance from the superconducting to a normally conducting state and the potential that the device can be made reusable. Some superconducting limiters, operating at voltages lower than that of interest to the power utilities, have also been described in the literature.
Series limiters have the limitation that they must operate at the operating potential. For passive devices, where there is adequate room in the utility switch yard, this is rarely a problem. However, if the device is switchable, then a high voltage transformer-like structure must be built for the trigger signals and the associated electronics. This adds to the complexity and cost of the limiter and active series limiters can also have difficulty in failing "safely." That is, they may not have sufficient heat capacity to prevent melting if a current switch fails to open in the allotted time.
For inductive limiters, the effect of the series inductance during operation needs to be minimal. If the value is small enough so that only a small amount of voltage appears across the inductor during normal operation, there is likely to be only a modest current limiting, for example, after the operating current is greater than ten times its normal value. In addition, there is no capacity to trigger a normal metal inductive limiter.
The current limiting fuse can cause a voltage spike of two or three times the system voltage when the fuse opens. Thus, the insulation required in the system must be adequate to handle the spike. Also, the fuse operates only once before replacement, and replacement can be a time consuming process. The vaporization of melted metal in fuses also limits the upper voltage at which they can be used.
Accordingly, an object of the present invention is an apparatus for limiting current in current sensitive components of a utility distribution process in which the current limiter limits currents to between three and ten times the normal operating current using a triggerable apparatus. Other objects of the invention are an economical, modest size, and reusable method and apparatus for limiting current in power utility switch gear. Yet another object of the invention is a fast-acting method and apparatus which can be added to existing switch gear equipment in a simple and economical manner for limiting current there through.